A two-wire phase control dimmer is configured to control a phase of AC voltage by a triac, and is used mainly for lump dimming or the like. This dimmer is designed assuming that an incandescent bulb is a load, and configured to determine an on-phase of the triac by a phase control circuit which operates in series connection to a filament of the incandescent bulb as the load for turning on the triac. However, this dimmer is commonly used, and thus it is preferred that this can be used for light dimming of an alternative incandescent bulb type LED lump or an alternative incandescent bulb type fluorescent lump other than the incandescent bulb. These lumps incorporate a lighting circuit including mainly a switching power supply such as a DC chopper and an inverter, and thus express an impedance different from the incandescent bulb.
However, the above dimmer determines the on-phase of the triac on the basis of an inputted value, thereby when the dimmer is series-connected to the incandescent bulb type LED or the incandescent bulb type fluorescent lump, the dimmer is affected by the impedance of these lumps. In order to operate normally the dimmer, in determining a dimming phase angle, it is necessary for the load impedance of the incandescent bulb type LED or the incandescent bulb type fluorescent lump series-connected to the phase control circuit to be stably low. When the load impedance is low, a circuit current is low in operation of the phase control circuit of the above dimmer, thereby there is approximately no problem as long as the load impedance is low in some degree (approximately a few hundreds Ω). On the contrary, if the load impedance varies or is a high impedance, a required circuit current for the phase control circuit of the dimmer cannot be acquired, thereby the power supply cannot be turned on at a desired phase. As a result, failures such as brightness flicker or non-lighting occur.
The above dimmer incorporates the triac being turned off during the off-phase, and thus a load current does not pass. At this time, the switching power supply incorporated in the load has generally a high impedance. However, the incorporated lightning circuit is connected to a noise prevention circuit or a smoothing capacitor, therefore if the supply is connected to the above dimmer as is, a failure occurs because of the unstable impedance.
Furthermore, when the phase control of the above dimmer is turned on, if the switching power supply incorporated in the load does not start up quickly in response to the turning on, on/off operation, turning off followed by turning on again and off, namely chattering may be repeated, because the triac is unable of holding the on-state.
In order to avoid the problem described above, it is possible to stabilize the operation by performing an action adding a changeover circuit causing the load impedance to lower or the like during the phase when the triac of the above dimmer is turned off. A dummy load made of a resistance can be also connected in parallel to an input side of the incandescent bulb type LED or the incandescent bulb type fluorescent lump or the like. Furthermore, the start up circuit can be also devised so as to start up quickly the switching power supply.
However, in the conventional techniques described above, because the phase control type dimmer such as a two-wire control dimmer is used, a circuit configuration for enabling dimming of lumps such as an incandescent bulb type LED lump or an incandescent bulb type fluorescent lump is complicated.
The invention provides a switching power supply device and an adjustable power supply system including the device. In the device, a special additional circuit is not necessary for adjusting a load by using the phase control circuit and the circuit configuration is simplified.